Without limiting the scope of the present invention, its background will be described with reference to producing fluid from a subterranean formation, as an example.
After drilling each of the sections of a subterranean wellbore, individual lengths of relatively large diameter metal tubulars are typically secured together to form a casing string that is positioned within each section of the wellbore. This casing string is used to increase the integrity of the wellbore by preventing the wall of the hole from caving in. In addition, the casing string prevents movement of fluids from one formation to another formation.
Conventionally, each section of the casing string is cemented within the wellbore before the next section of the wellbore is drilled. Accordingly, each subsequent section of the wellbore must have a diameter that is smaller than the previous section. For example, a first section of the wellbore may receive a conductor casing string having a 20-inch diameter. The next several sections of the wellbore may receive intermediate casing strings having 16-inch, 13⅜-inch and 9⅝-inch diameters, respectively. The final sections of the wellbore may receive production casing strings having 7-inch and 4½-inch diameters, respectively. Each of the casing strings may be hung from a casinghead near the surface. Alternatively, some of the casing strings may be in the form of liner strings that extend from near the setting depth of previous section of casing. In this case, the liner string will be suspended from the previous section of casing on a liner hanger. Additionally, as should be understood by those skilled in the art, other techniques could be used to construct the wellbore including using a monobore casing design, casing drilling techniques, expandable tubulars or the like.
Once this well construction process is finished, the completion process may begin. For example, the completion process may include creating hydraulic openings or perforations through the production casing string, the cement and a short distance into the desired formation or formations so that production fluids may enter the interior of the wellbore. In addition, the completion process may involve one or more treatment processes such as formation stimulation to enhance production, gravel packing to prevent sand production or the like. The completion process also includes installing a production tubing string within the well that extends from the surface to the production interval or intervals.
Unlike the casing strings that form a part of the wellbore itself, the production tubing string is used to produce the well by providing the conduit for formation fluids to travel from the formation depth to the surface. In addition, tools within the tubing string provide for the control of the fluids being produced from the formation. For example, the production tubing string typically includes one or more seal assemblies. The seal assemblies may be installed above and below each production interval to isolate the production from each interval. Once a seal assembly is properly located within the wellbore, the seal assembly is actuated to create a sealing and gripping relationship with the walls of the adjacent casing or liner. As such, the seal assembly provides a seal in the annular space between the production tubing and the casing to prevent fluid flow and contain pressure.
To achieve the gripping relationship, typical seal assemblies are equipped with anchor slips that have opposed camming surfaces that cooperate with complementary opposed wedging surfaces. The anchor slips are radially extendable into gripping engagement against the well casing bore in response to relative axial movement of the wedging surfaces. To achieve the sealing relationship, typical seal assemblies carry annular seal elements that are expandable radially into sealing engagement against the bore of the well casing in response to an axial compression force. Mechanical or hydraulic means typically may be used to set the anchor slips and the sealing elements. For example, the mechanically set seal assemblies may be actuated by pipe string rotation or reciprocation. Alternatively, mechanically set seal assemblies may be actuated by employing a setting tool that is run downhole and coupled to the seal assembly for setting. Likewise, hydraulically set seal assemblies may be actuated using a setting tool that is run downhole and coupled in fluid communication with the seal assembly. Alternatively, elevating the fluid pressure within the tubing string may be used to actuate hydraulically set seal assemblies.
It has been found, however, that due to the long service life and high pressures operating against seal assemblies, some seal assemblies may move within the wellbore over the course of time. Such movement may be an indication that the seal assembly is about to fail. In addition, it has been found that this movement may be too slow to detect using conventional measurement techniques such as through the use of accelerometers. While accelerometers are useful in detecting fast movements, movement below a certain threshold will go undetected.
Therefore a need has arisen for an apparatus and method for detecting movement of a seal assembly once the seal assembly has been installed within a wellbore and before the seal assembly fails. A need has also arisen for such an apparatus and method that are capable of detecting slow movement of a seal assembly including movement below the threshold detectable by an accelerometer.